Replication-defective mammalian transforming retroviruses appear to have been arisen by recombination between the genome of type C (helper) viruses and host cellular sequences. In order to identify translational products encoded by such acquired cellular sequences, genomic RNAs of a number of recombinant viruses have been purified and translated in Xenopus laevis oocytes. Characterization of resulting translational products has been achieved by immunological and biochemical methods.